1. Field of the Invention
The present invention relates to an optical CATV system and, more particularly, to an optical CATV system having enhanced transmission efficiency of communication channels to offer voice telephone and narrow-band ISDN services over one E1 (2.048 Mbps) frame between subscriber units and a distribution center.
2. Discussion of Related Art
In general, as shown in FIG. 3, CATV system comprises a transmit facility such as head end, a transmission facility such as cables, interexchange amplifiers or the like, and a terminal facility connected to terminals such as television and telephone. U.S. Pat. No. 5,600,656 discloses a typical CATV system, which converts a plurality of broad-band satellite channel signals into proper ones for a corresponding receiver to transmit them over a plurality of channels. It is thus designed to interface a plurality of channel signals through one system.
The optical CATV system uses optical fibers as cables in the transmission facility for transmitting TV/stereo audio signals, voice telephone network service and narrow-band ISDN communication service to subscribers. As compared with conventional telephone wire or coaxial cable, the optical fibers have several advantages: their broad bandwidth results in a low cost per channel, high reliability for data transmission, and low error rate. The optical fibers are also capable of being installed at a low costs because it is small in size and light-weighted.
The subscriber unit of the optical CATV system affords to three subscribers, according to the regulations, allotting three channels to one subscriber for the voice telephone (B-CH) service and four time slots of an E1 frame for the narrow-band ISDN service.
Light signals of 155 Mbps received between the distribution center DC for distributing the services to each subscriber and the subscriber terminals are converted into electrical signals and demultiplexed to separately offer the services. The light signals of 155 Mbps are allotted by E1 system for the voice telephone and narrow-band ISDN services, serving nine channels for a voice telephone and channels for three subscribers for a narrow-band ISDN.
E1 (2.048 Mbps) system has 30 available channels to offer the voice telephone and narrow-band ISDN services respectively over an E1 frame. Therefore, two E1 (2.048 Mbps) lines per one subscriber are used for the two services. FIG. 4 is a format diagram of a transmission frame allotted for the voice telephone and narrow-band ISDN. Referring to FIG. 4, the voice telephone data is transmitted over one of the 1st to 3rd time slots of the E1 frame and the narrow-band ISDN data is transmitted over the 1st to 4th time slots of the E1 frame.
Since two E1 lines are allotted for one subscriber to offer the voice telephone and narrow-band ISDN services, the voice telephone data is transmitted over one of 32 channels and the ISDN data is transmitted over four channels of the 32 channels, thereby using the channels inefficiently and deteriorating the transmission efficiency.